The invention relates to a method and device for detecting compounds in a gas stream, wherein the gas stream is irradiated in an ionization chamber of a mass spectrometer by an UV laser pulse and the ions generated thereby are detected in the mass spectrometer.
1. State of the Art
The resonance-enhanced multi-photon ionization (REMPI) technique, which utilizes UV-laser pulses for a selective ionization of for example aromatics, is used as a selective and soft ionization method for the mass spectrometry. The selectivity is determined among others by the UV spectroscopic properties and the location of the ionization potentials. A typical application is the on-line detection of aromatic compounds in exhaust gases1. It is a disadvantage of the REMPI method that it is limited to several substance classes and that the ionization cross-section may sometimes be very different for similar compounds.
The single photon ionizationxe2x80x94(SPI) with VUV laser light permits a partially selective and soft ionization2.
The selectivity is determined by the location of the ionization potentials. A typical application is the detection of compounds, which cannot be detected by REMPI. A disadvantage with the SPI method however is that some substance classes cannot be detected. Furthermore, the selectivity is smaller than with the REMPI method so that, with complex samples, interferences can be strong.
The electron impulse ionization (EI) using an electron beam is the standard technique for the ionization in the mass spectrometry of volatile organic and inorganic compounds. It is very universal (that is, not selective) and, with many molecules, results in a high fragmentation. However, it is highly suitable for a direct detection of compounds such as O2, N2, CO2, C2H2, etc, which cannot be well detected by VUV or REMPI.
2. Object of the Invention
It is the object of the invention to provide a method and a device of the type referred to above with which however a multitude of compounds in the gas to be analyzed can be detected almost at the same time.
In a method and apparatus for detecting compounds in a gas stream, the gas stream with the compounds to be detected is conducted into an ionization chamber of a mass spectrometer where the gas stream is subjected in the ion chamber in a pulsed manner alternately to UV laser pulses and to vacuum ultraviolet VUV laser pulses and the ions generated thereby are directed into the mass spectrometer for detection therein to determine the compounds in the gas stream.
The combination of SPI and REMPI ionization performed in a mass spectrometer (quasi) simultaneously has a number of advantages. Both methods detect different partial amounts of the complex analysis gases with a different selectivity. In this way, altogether, more compounds of a sample can be identified.
If also the EI-ionization technique is utilized, additional compounds such as CO2, H2O or Ch4 can be detected, which cannot reasonably be detected with SPI or with REMPI. The combination of the methods and the device for the quasi-parallel use of the methods in a single apparatus results in the construction of particularly compact analytical MS-systems for example for online analytical field surveillance (process analysis), which have a very high performance. The REMPIxe2x80x94and/or VUVxe2x80x94and/or EI mass spectrometric data obtained in a parallel process may also be supplied to a chemometric analysis by way of sample recognition procedures (for example, a main component analysis).